Muhammad Ali has always been known for his larger-than-life personality, but Bill Siegel’s documentary “The Trials of Muhammad Ali” charts how the young boxing champion evolved into a political activist.

“Trials” covers the 1960s, when Ali changed his name from Cassius Clay and came under fire for refusing to go to Vietnam. Supporters and observers share their impressions of his legal battles, and the fighter speaks for himself through riveting archival footage.

Siegel (“The Weather Underground”) details the metamorphosis with honesty, noting the champ’s failures alongside his triumphs. The emergent image is of a man who valued his convictions above all else, as tenacious and bold in his personal life as he was in the ring.